Why doesn't my GPU work for my mac?
Please make sure you have installed the CUDA driver for your mac:
http://www.nvidia.com/object/mac-driver-archive.html

I have a 750M in my MacBook Pro (2.8 GHz Intel Core i7) and it renders about as fast as the CPU as long as the render job's RAM requirement doesn't exceed the RAM on the video card. In fact, I often render overnight with both CPU and GPU concurrently as separate command lines.